The present invention relates to a retractor (a wind-up apparatus) for a seat belt and, in particular, to a retractor for a seat belt having an energy absorption mechanism.
Conventionally, as a retractor for a seat belt which is used to hold a passenger or the like of a vehicle safely with respect to a seat of the vehicle, there is used a retractor of an emergency lock type which includes an emergency lock mechanism for locking the retractor physically by means of inertia sensing means capable of responding to a rapid acceleration, a collision, or a rapid deceleration, thereby being able to restrain the passenger effectively and safely.
On the other hand, when a shock due to a collision is extremely great, the tension of a webbing increases with the passage of time after the collision to thereby cause a rapid deceleration in the body of the passenger, so that the load to be applied from the webbing onto the passenger becomes extremely large. In view of this, conventionally, there have been proposed various types of seat belt devices each including an energy absorption mechanism which, when the load to be applied onto the webbing becomes equal to or more than a predetermined value which has been previously set, causes a seat belt to be played out a predetermined amount to thereby absorb the shock that is produced in the body of the passenger to thereby be able to protect the body of the passenger against the collision shock positively. As the thus structured retractor for use in the seat belt, there is known "An energy absorption device especially for use in a safety belt" which is disclosed in Japanese Patent Unexamined Publication No. Sho. 46-7710.
The above-mentioned energy absorption device comprises a winding member (that is, a bobbin) to which the energy absorption device transmits a force, a holder (that is, a retractor base) which can be rotated relatively with respect to the winding member, and a torsion bar (that is, a twist rod) which is interposed between the holder and winding member. In a vehicle emergency, a gear (that is, a locking member) disposed in one end portion of the torsion bar is secured to a securing lever and is thereby connected to the holder in an unrotatable manner. On the other hand, the other end portion of the torsion bar is previously connected with the winding member in an unrotatable manner. For this reason, when a torque in a webbing drawn-out direction is further applied to the winding member, the torsion bar is twisted about its own axis and is thus deformed plastically, so that the vehicle emergency energy such as the collision energy acting on the body of the passenger can be absorbed as the deformation work of the torsion bar which, as described above, serves as a plastically deforming member.
However, in this case, the amount of twisting or torsional deformation of the torsion bar (namely, the energy absorption stroke thereof) is not restricted and, in a state that the torsional deformation amount is not restricted, the amount of drawn-out of the webbing is excessively large, which increases the possibility that, in a vehicle having a small indoor space, the passenger is subjected to a so-called secondary collision.
In order to avoid this, as a retractor for use in a seat belt comprising an energy absorption mechanism which includes stopper members for restricting the energy absorption stroke, for example, there is proposed a retractor under the name of "A wind-up apparatus for use in a safety belt" which is disclosed in Japanese Patent Examined Publication No. Sho. 57-6948.
In the energy absorption mechanism employed in the above-disclosed wind-up apparatus, to a latch plate (which serves as a locking member) mounted on a webbing winding shaft, there is fixed a first member which is arranged coaxial with the webbing winding shaft, and a second member is fixed to the webbing winding shaft in such a manner that it is opposed to the first member, while there is mounted a rolling body (which serves as plastically deforming member) on a groove formed in at least one of the first and second members. That is, when the rolling body is rolled on the groove, the groove is plastically deformed by the movement of the rolling body, so that the collision energy acting on the body of the passenger can be absorbed as the plastical deformation work of the groove. In addition, as a stopper mechanism, a key is planted in the neighborhood of the end of the above groove; that is, the advance of the rolling body is stopped by the key thus implanted, thereby being able to restrict the energy absorption stroke.
However, in the above-mentioned stopper mechanism using the key, when the stopper mechanism is in operation, a large load is applied to the key planted in the groove and, therefore, the key must maintain a sufficient stopper strength. In addition, in the above-mentioned stopper mechanism, when the energy absorption is ended, the rotation of the webbing winding shaft is stopped by the latch plate locked to the retractor base. This stopping operation makes it necessary that the above-mentioned rolling body as well as the above-mentioned first and second members existing between the latch plate and webbing winding shaft respectively should have a final strength, which can endure all of the tension forces finally applied to the webbing.
That is, in order to be able to restrain the passenger for sure in a vehicle emergency, all of the parts existing between the above-mentioned latch plate and the above-mentioned webbing winding shaft must have a sufficient strength and, therefore, they must be made of expensive materials and must be produced according to a complicated working method, so that the production costs of these parts are high.